I too am having the same problem.
Ubuntu 10.4
Wine from ubuntu repositories (forget version)
ATI Radeon Mobility 1 GB HD 4670
6 GB RAM
Using Catalyst 10.6 drivers.
While I don't expect the FPS to be the same at all .. I figured it would at least be only a bit of a loss ...
As an unfair comparison .. I usually get about 60 FPS in a city like Stormwind under Windows ... under wine I can barely get 30 .. I expect a 10% loss .. but 50% seems a bit drastic to me.
Odd thing is I have followed all tips about turning off effects, reducing my resolution .. you name it .. still no change.